Temperature Compensating Transparent Force Sensor Having a Flexible Substrate

ABSTRACT

An optically transparent force sensor element is compensated for effects of environment by comparing a force reading from a first force-sensitive component with a second force-sensitive components. The first and second force-sensitive components disposed on opposite sides of a flexible substrate within a display stack.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 14/594,857, filed Jan. 12, 2015, and titled “Temperature Compensating Transparent Force Sensor Having a Flexible Substrate,” which claims priority to U.S. Provisional Patent Application No. 61/926,905, filed Jan. 13, 2014, and titled “Force Sensor Using a Transparent Force-Sensitive Film,” U.S. Provisional Patent Application No. 61/937,465, filed Feb. 7, 2014, and titled “Temperature Compensating Transparent Force Sensor,” U.S. Provisional Patent Application No. 61/939,257, filed Feb. 12, 2014, and titled “Temperature Compensating Transparent Force Sensor,” U.S. Provisional Patent Application No. 61/942,021, filed Feb. 19, 2014, and titled “Multi-Layer Temperature Compensating Transparent Force Sensor,” and U.S. Provisional Patent Application No. 62/024,566, filed Jul. 15, 2014, and titled “Strain-Based Transparent Force Sensor,” the disclosure of each of which is incorporated by reference in its entirety.

TECHNICAL FIELD

Embodiments described herein generally relate to force sensing and, more particularly, to a temperature compensating force sensor having two or more transparent force-sensitive components disposed on either side of a flexible substrate.

BACKGROUND

Many electronic devices include some type of user input device, including, for example, buttons, slides, scroll wheels, and similar devices or user-input elements. Some devices may include a touch sensor that is integrated or incorporated with a display screen. The touch sensor may allow a user to interact directly with user-interface elements that are presented on the display screen. However, some traditional touch sensors may only provide a location of a touch on the device. Other than location of the touch, many traditional touch sensors produce an output that is binary in nature. That is, the touch is present or it is not.

In some cases, it may be advantageous to detect and measure the force of a touch that is applied to a surface to provide non-binary touch input. However, there may be several challenges associated with implementing a force sensor in an electronic device. For example, temperature fluctuations in the device or environment may introduce an unacceptable amount of variability in the force measurements. Additionally, if the force sensor is incorporated with a display or transparent medium, it may be challenging to achieve both sensing performance and optical performance in a compact form factor.

SUMMARY

Embodiments described herein may relate to, include, or take the form of an optically transparent force sensor, which may be used as input to an electronic device. The optically transparent force sensor may be configured to compensate for variations in temperature using two or more force-sensitive structures that are disposed on opposite sides of a flexible substrate.

In some example embodiments, an optically transparent force sensor includes, a force-receiving layer and a substrate comprising an optically transparent material, the substrate disposed below the force-receiving layer. The force sensor may also include a first force-sensitive component disposed on a first side of the substrate and comprised of an optically transparent, strain-sensitive material, and a second force-sensitive component disposed on a second side of the substrate that is opposite to the first side. The second force-sensitive component is also comprised of the optically transparent, strain-sensitive material.

In some embodiments, the sensor also includes sensor circuitry that is operatively coupled to the first force-sensitive component and the second force-sensitive component. In some implementations the sensor circuitry is configured to measure a relative difference between an electrical response of the first and second force-sensitive components in response to a force of a touch on the force-receiving layer. The sensor circuitry may also be configured to compute a temperature-compensated force estimate using the relative difference.

In some implementations, the substrate is configured to conduct heat between the first force-sensitive component and the second force-sensitive component to achieve a substantially uniform temperature distribution. In some cases, the substrate has a thermal conductivity of greater than 0.5 Watts per square meter per degree Kelvin. In some cases, the first and second force-sensitive components have substantially identical temperature coefficients of resistance.

The transparent force sensor may be integrated or incorporated with a display element. In some cases, the substrate is disposed below a display element of an electronic device. In some cases, the substrate is disposed between a cover and a display element of an electronic device. In some cases, the force-receiving layer is a cover of a display of a device and is formed from a glass material.

In some embodiments, the first force-sensitive component is placed in compression in response to a force of a touch on the force-receiving layer, and the second force-sensitive component is placed in tension in response to the force of the touch. In some embodiments, the sensor includes a first array of rectilinear force-sensitive components including the first force-sensitive component and a second array of rectilinear force-sensitive components including the second force-sensitive component.

In some embodiments, the first and second force-sensitive components are formed from a piezoresistive material. In some embodiments, the first and second force-sensitive components are formed from one or more of: a carbon nanotube material, graphene, gallium zinc oxide, indium gallium zinc oxide, a semiconductor material, a metal oxide material. In some embodiments, the first and second force-sensitive components are formed from a indium oxide material that is doped with Sn. The indium oxide material may be doped with Sn to a proportion of less than 5%.

Some example embodiments are directed to an electronic device having an optically transparent force sensor. The electronic device may include a cover and a substrate comprising an optically transparent material, the substrate disposed below the cover The device may also include a first array of force-sensitive components disposed on a first side of the substrate and comprised of an optically transparent, strain-sensitive material. The device may also include a second array of force-sensitive components disposed on a second side of the substrate that is opposite to the first side, wherein the second array of force-sensitive components is comprised of the optically transparent, strain-sensitive material. In some cases, the device includes sensor circuitry that is configured to compare a relative electrical response between respective components of the first array of force-sensitive components and the second array of force-sensitive components, and configured to compute a temperature-compensated force estimate. The device may also include a display element disposed above the first array of force-sensitive components. In some cases the display element is disposed below the second array of force-sensitive components.

In some embodiments, the first array of force-sensitive components includes a subset of edge force-sensitive components that are positioned along an edge of the first array. In some cases, the edge force-sensitive components are formed from traces that are oriented along a direction that is substantially perpendicular to the edge. In some embodiments, the first array of force-sensitive components includes a subset of corner force-sensitive components positioned at corners of the first array. The corner force-sensitive components may be formed from traces that are oriented along a diagonal direction. In some embodiments, the first array of force-sensitive components includes component having a first portion that includes traces that are substantially oriented along a first direction and a second portion that includes traces that are substantially oriented along an second direction. The first direction may be substantially perpendicular to the second direction.

Other embodiments described herein may relate to, include, or take the form of a method of manufacturing a force sensor including at least the steps of applying a first force-sensitive film to a first substrate, and applying a second force-sensitive film to the first substrate or a second substrate resulting in a force sensor having the first force-sensitive film and the second force sensitive film disposed on opposite sides of the first substrate.

BRIEF DESCRIPTION OF THE FIGURES

Reference will now be made to representative embodiments illustrated in the accompanying figures. It should be understood that the following descriptions are not intended to limit the embodiments to one preferred embodiment. To the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the described embodiments as defined by the appended claims.

FIG. 1 depicts an example electronic device.

FIG. 2A depicts a top view of an example force-sensitive structure including a grid of optically transparent force-sensitive films.

FIG. 2B depicts a top detailed view of an optically transparent serpentine force-sensitive film which may be used in the example force-sensitive structure depicted in FIG. 2A.

FIG. 3 depicts a cross-sectional view of a portion of the example force-sensitive structure of FIG. 1 taken along section A-A.

FIG. 4 depicts a top view of an alternate example of a force-sensitive structure including two perpendicular layers each including multiple optically transparent force-sensitive films.

FIG. 5A depicts a top detailed view of an optically transparent serpentine force-sensitive component having a first serpentine pattern and which may be used in the example force-sensitive structure depicted in FIG. 2A.

FIG. 5B depicts a top detailed view of an optically transparent serpentine force-sensitive component having a second serpentine pattern and which may be used in the example force-sensitive structure depicted in FIG. 2A.

FIG. 5C depicts a top detailed view of an optically transparent serpentine force-sensitive components having a third serpentine pattern and which may be used in the example force-sensitive structure depicted in FIG. 2A.

FIG. 5D depicts a top detailed view of an optically transparent serpentine force-sensitive component having a fourth serpentine pattern and which may be used in the example force-sensitive structure depicted in FIG. 2A.

FIG. 6 depicts a top view of an example force-sensitive structure including a grid of optically transparent force-sensitive components oriented in different directions to detect force.

FIG. 7A depicts a top detailed view of an optically transparent serpentine force-sensitive component having a fifth serpentine pattern and which may be used in the example force-sensitive structure depicted in FIG. 2A.

FIG. 7B depicts a top view of an example force-sensitive structure including a grid of optically transparent force-sensitive components to detect force.

FIG. 8 depicts a simplified signal flow diagram of a temperature-compensating and optically transparent force sensor circuit.

FIG. 9 is a process flow diagram illustrating example steps of a method of manufacturing a temperature-compensating and optically transparent force sensor.

FIG. 10 is a process flow diagram illustrating example steps of a method of operating a temperature-compensating force sensor.

FIG. 11 is an additional process flow diagram illustrating example steps of a method of operating a temperature-compensating force sensor.

The use of the same or similar reference numerals in different figures indicates similar, related, or identical items.

DETAILED DESCRIPTION

Embodiments described herein may relate to or take the form of temperature compensating optically transparent force sensors for receiving user input to an electronic device. Some embodiments are directed to a force sensor that can compensate for variations in temperature and may be optically transparent for integration with a display or transparent medium of an electronic device. Some embodiments relate to force-sensitive structures including one or more force-sensitive components for detecting a magnitude of a force applied to a device. In one example, a transparent force-sensitive component is integrated with, or adjacent to, a display element of an electronic device. The electronic device may be, for example, a mobile phone, a tablet computing device, a computer display, a computing input device (such as a touch pad, keyboard, or mouse), a wearable device, a health monitor device, a sports accessory device, and so on.

Generally and broadly, a user touch event may be sensed on a display, enclosure, or other surface associated with an electronic device using a force sensor adapted to determine the magnitude of force of the touch event. The determined magnitude of force may be used as an input signal, input data, or other input information to the electronic device. In one example, a high force input event may be interpreted differently from a low force input event. For example, a smart phone may unlock a display screen with a high force input event and may pause audio output for a low force input event. The device's responses or outputs may thus differ in response to the two inputs, even though they occur at the same point and may use the same input device. In further examples, a change in force may be interpreted as an additional type of input event. For example, a user may hold a wearable device force sensor proximate to an artery in order to evaluate blood pressure or heart rate. One may appreciate that a force sensor may be used for collecting a variety of user inputs.

In many examples, a force sensor may be incorporated into a touch-sensitive electronic device and located proximate to a display of the device, or incorporated into a display stack. Accordingly, in some embodiments, the force sensor may be constructed of optically transparent materials. For example, an optically transparent force sensor may include at least a force-receiving layer, a first and second substrate each including at least an optically transparent material, and each substrate including, respectively, a first and second force-sensitive component. In some embodiments, the first and second force-sensitive components are disposed, or located relative to, opposite sides of the first substrate. In some cases, the first and second force-sensitive components are formed on or attached to the first substrate. In other cases, either or both of the first and second force-sensitive components may be formed on or attached to a second substrate. In many examples, the first substrate may be disposed below the force-receiving layer such that the first force-sensitive component may experience deflection, compression, or another mechanical deformation upon application of force to the force-receiving layer. In this manner, a bottom surface of the first substrate may experience an expansion and a top surface of the first substrate may experience a compression. In other words, the first substrate may bend about its neutral axis, experiencing compressive and tensile forces. The force-sensitive components may be used to detect and measure the degree of expansion and compression caused by the deflection of the first substrate.

A transparent force-sensitive component is typically a compliant material that exhibits at least one electrical property that is variable in response to deformation, deflection, or shearing of the component. The transparent force-sensitive component may be formed from a piezoelectric, piezoresistive, resistive, or other strain-sensitive materials. Potential substrate materials include, for example, glass, sapphire, diamond, SiO₂, or transparent polymers like polyethylene terephthalate (PET) or cyclo-olefin polymer (COP). Example transparent conductive materials include polyethyleneioxythiophene (PEDOT), indium tin oxide (ITO), carbon nanotubes, graphene, gallium zinc oxide, indium gallium zinc oxide, other doped or undated metal oxides, piezoresistive semiconductor materials, piezoresistive metal materials, silver nanowire, platinum nanowire, nickel nanowire, other metallic nanowires, and the like. Transparent materials may be used in sensors that are integrated or incorporated with a display or other visual element of a device. If transparency is not required, then other component materials may be used, including, for example, Constantan and Karma alloys, doped polycrystalline or amorphous silicon, or single crystal silicon or other semiconductor material for the conductive component and a clad metal, ceramic, or a polyimide may be used as a substrate. Nontransparent applications include force sensing on track pads or behind display elements. In general, transparent and non-transparent force-sensitive components may be referred to herein as “force-sensitive components” or simply “components.”

Transparent force-sensitive components can be formed by coating a substrate with a transparent conductive material, attaching a transparent conductive material, or otherwise depositing such a material on the substrate. In some embodiments, the force-sensitive components may be formed relative to the bottom surface of a first substrate and relative to a top surface of a second substrate. The force-sensitive components of the first and second substrates may be oriented to face one another. In some implementations, the first substrate may deflect in response to a user touch. The deflection of the first substrate may cause the bottom surface of the first substrate to expand under tension, which may cause the transparent force-sensitive component (disposed relative to the bottom surface) to also expand, stretch, or otherwise geometrically change as a result of the deflection.

In some cases, the force-sensitive component may be placed under tension in response to a downward deflection because the component is positioned below the neutral axis of the bend of the substrate. Once under tension, the transparent force-sensitive component may exhibit a change in at least one electrical property, for example, resistance. In one example, the resistance of the transparent force-sensitive component may increase linearly with an increase in tension experienced by the component. In another example, the resistance of the transparent force-sensitive component may decrease linearly with an increase in tension experienced by the component. One may appreciate that different transparent materials may experience different changes to different electrical properties, and as such, the effects of tension may vary from embodiment to embodiment.

As previously mentioned, suitable transparent conductive materials include, for example, polyethyleneioxythiophene (PEDOT), carbon nanotubes, graphene, silver nanowire, other metallic nanowires, and the like. In some cases, the transparent conductive material may include a metal oxide material, including, for example, SnO₂, In₂O₃, ZnO, Ga₂O₃, and CdO. The transparent conductive material may also be formed from an indium oxide material. In some cases, the indium oxide is doped with tin (Sn) to form an indium-tin oxide. In some implementations, the indium oxide is doped with Sn to a proportion of less than 5%. Additionally, in some cases, the transparent conductive material may be formed from a semiconductor material, including, for example, a piezoresistive semiconductor material. Potential substrate materials include, for example, glass or transparent polymers like polyethylene terephthalate (PET) or cyclo-olefin polymer (COP). Typically, when a piezoresistive or resistive component is strained, the resistance of the component changes as a function of the strain. The resistance can be measured with an electrical circuit.

In some embodiments, the force-sensitive components may be formed from a piezoresistive or resistive material. In some implementations, when the piezoresistive or resistive material is strained, the resistance of the component changes as a function of the strain. The change in resistance can be measured using a sensing circuit that is configured to measure small changes in resistance of the force-sensitive components. In some cases, the sensing circuit may include a bridge circuit configuration that is configured to measure the differential change in resistance between two or more force-sensitive components. If the relationship between electrical resistance, temperature and mechanical strain of the component material is known, the change in the differential strain ε_(x)-ε_(y) may be derived. In some cases, the differential strain may account for changes strain or resistance due to changes in temperature, which may cancel if the two elements have similar thermal properties and are at similar temperature while being subjected to differential strain due to a placement with respect to the neutral axis of a flexible substrate. In this way, a transparent piezoresistive or resistive component can be used as a temperature compensating force sensor.

In certain embodiments, a resistive element may be measured by using a voltage divider or bridge circuit. For example, a voltage V_(g) may be measured across the output of two parallel voltage dividers connected to a voltage supply V_(s). One of the voltage dividers may include two resistors of known resistance R₁ and R₂, the other voltage divider may include a first resistive strain element R_(x) and a second resistive strain element R_(y). A voltage can be measured between a node between R₁ and R₂ and a node between R_(x) and R_(y) to detect small changes in the relative resistance between the two strain elements. In some cases, additional sensor circuitry (including a processing unit) may be used to calculate the mechanical strain due to a force on the surface based on the relative resistance between two strain elements. In some cases, the sensor circuitry may estimate the mechanical strain while reducing or eliminating environmental effects, such as variations in temperature.

In some embodiments, pairs of voltage dividers may be used to form a full bridge, so as to compare the output of a plurality of sensors. In this manner, error present as a result of temperature differences between sensors may be substantially reduced or eliminated without requiring dedicated error correction circuitry or specialized processing software. In some embodiments, an electrical response due to the force of a touch may be measured and an algorithm may be used to compare a relative response and cancel the effects of the temperature changes. In some embodiments, both differential measurements of the components and measurements of their individual responses may be made to extract the corresponding differential strain, and also the temperature. In some cases an algorithm may use the differential and individual responses to compute a force estimate that cancels the effects on strain due to, for example, the differences in the thermal coefficient of expansion of the two component materials.

In some embodiments, the force-sensitive component is patterned into an array of lines, pixels, or other geometric elements herein referred to as “component elements.” The regions of the force-sensitive component or the component elements may also be connected to sense circuitry using electrically conductive traces or electrodes. In some cases, the conductive traces or electrodes are also formed from transparent conductive materials. In some embodiments, sense circuitry, may be in electrical communication with the one or more component elements via the electrically conductive traces and/or the electrodes. As previously mentioned, the sense circuitry may be adapted to detect and measure the change in the electrical property or response (e.g., resistance) of the component due to the force applied.

In some cases, the force-sensitive components may be patterned into pixel elements, each pixel element including an array of traces generally oriented along one direction. This configuration may be referred to as a piezoresistive or resistive strain gauge configuration. In general, in this configuration the force-sensitive-component may be composed of a material whose resistance changes in a known fashion in response to strain. For example, some materials may exhibit a change in resistance linearly in response to strain. Some materials may exhibit a change in resistance logarithmically or exponentially in response to strain. Some materials may exhibit a change in resistance in a different manner. For example, the change in resistance may be due to a change in the geometry resulting from the applied strain such as an increase in length combined with decrease in cross-sectional area may occur in accordance with Poisson's effect. The change in resistance may also be due to a change in the inherent resistivity of the material due to the applied strain.

In some embodiments, the orientation of the strain-sensitive elements may vary from one part of the array to another. For example, elements in the corners may have traces that are oriented to be sensitive to strain at 45 degrees with respect to a row (or column) of the array. Similarly, elements along the edge of the array may include traces that are most sensitive to strain perpendicular to the edge or boundary. In some cases, elements may include one of a variety of serpentine trace configurations that may be configured to be sensitive to a combination of the strains along multiple axes. The orientation of the traces in the strain-sensitive elements may have different angles, depending on the embodiment.

The pixel elements may have trace patterns that are configured to blend the sensitivity to strain along multiple axes to detect changes in boundary conditions of the sensor or damage to the device. For example, if an element, component, or substrate becomes less constrained because of damage to the physical edge of a device, the sensitivity of the response to strain in the X direction may become higher, while the sensitivity of the response to strain in the Y direction may be lower. However, if the pixel element is configured to be responsive to both X and Y directions, the combined response of the two or more directions (which may be a linear combination or otherwise) may facilitate use of the sensor, even after experiencing damage or changes in the boundary conditions of the substrate.

In some embodiments, the force-sensitive component may be formed from a solid sheet of material and may be placed in electrical communication with a pattern of electrodes disposed on one or more surfaces of the force-sensitive component. The electrodes may be used, for example, to electrically couple a region of the solid sheet of material to sense circuitry. An electrode configuration may be used to measure a charge response when strained. In some cases, the force-sensitive component may generate different amounts of charge depending on the degree of the strain. The overall total charge may reflect a superposition of the charge generated due to strain along various axes.

In some embodiments, the force-sensitive component may be integrated with, or placed adjacent to, portions of a display element, herein generally referred to as a “display stack” or simply a “stack.” A force-sensitive component may be integrated with a display stack, by, for example, being attached to a substrate or sheet that is attached to the display stack. In this manner, as the display stack bends in response to an applied force, and through all the layers which have good strain transmission below the neutral axis, a tensile strain is transmitted.

Alternatively, the force-sensitive component may be placed within the display stack in certain embodiments. Although certain examples are herein provided with respect to a force-sensitive component integrated with a display stack, in other embodiments, the force-sensitive component may be integrated in a portion of the device other than the display stack.

In some embodiments, one or more force-sensitive components may be integrated with or attached to a display element of a device, which may include other types of sensors. In some embodiments, a display element may include a touch sensor included to detect the location of one or more user touch events. Using a touch sensor in combination with the transparent force-sensitive component in accordance with some embodiments described herein, the location and magnitude of a touch on a display element of a device can be estimated.

In some embodiments, the device may include both touch-sensitive elements and force-sensitive elements relative to a surface that may cooperate to improve accuracy of the force sensors. In some cases, the information from the touch-sensitive elements may be used in combination with stored information about the responsiveness of the surface to reconstruct the force exerted on the surface. For example, the location determined by the touch sensor may be used in conjunction with a set of weighting coefficients stored in a memory to estimate the force applied at the corresponding points. A different touch location may be used in conjunction with a different set of coefficients weighting the response of the strain sensors to predict a force of touch at that point. In certain examples, the algorithm used to calculate the forces at the surface may be based, at least in part, upon the information provided by the touch sensor, stored information from calibration of the display, or information collected and stored during the operational life of the sensors. In some cases, the sensors may be calibrated to zero force during a time preceding a touch indication from the touch sensors.

One challenge associated with using a force-sensitive component or film within a display stack is that the given electrical property (for example, resistance) may change in response to temperature variations as the electronic device is transported from place to place, or used by a user. For example, each time a user touches the touch screen, the user may locally increase the temperature of the screen and force-sensitive component. In other examples, different environments (e.g., indoors or outdoors) may subject the electronic device to different ambient temperatures. In still further examples, an increase in temperature may occur as a result of heat produced by electronic components or systems of the device.

In some cases, the force-sensitive component may also expand and contract in response to changes in other environmental conditions, such as changes in humidity or barometric pressure. In the following examples, the electrical property is a resistance and the variable environmental condition is temperature. However, the techniques and methods described herein may also be applied to different electrical properties, such as capacitance or inductance, which may be affected by changes in other environmental conditions.

In some implementations, a change in temperature or other environmental conditions, either locally or globally, may result in expansion or contraction of the force-sensitive component, electronic device enclosure, and/or other components adjacent to the component which in turn may change the electrical property (e.g., resistance) measured by the sense circuitry. In many cases, the changes in the electrical property due to temperature change may obfuscate any changes in the electrical property as a result of an input force. For example, a deflection may produce a reduction or increase in the resistance or impedance of the force-sensitive component. A change in temperature may also produce a reduction or increase in the resistance or impedance of the force-sensitive component. As a result, the two effects may cancel each other out or, alternatively, may amplify each other resulting in an insensitive or hypersensitive force sensor. A similar reduction or increase in the resistance or impedance of the force-sensitive component could also be produced by, for example, an increase in temperature of the force-sensitive component due to heat produced by other elements of the device.

In some cases, mechanical changes due to variations in temperature may also impact the electrical performance of the sensor. In particular, variations in temperature of the force-sensing component may result in variations in strain on the force-sensing elements. For example, a heated force-sensitive component may expand and a cooled force-sensitive component may contract producing a strain on the component. This strain may cause a change in resistance, impedance, current, or voltage that may be measured by associated sense circuitry and may impact the performance of the force sensor.

One solution is to account for environmental effects by providing more than one force-sensing component that is subjected to the same or substantially the same environmental conditions. A first force-sensing component may serve as a reference point or environmental baseline while measuring the strain of a second force-sensing component. In some implementations, both of the force-sensitive components may be constructed of substantially identical materials such that the reference component reacts to the environment in the same manner as the component being measured. For example, in some cases, each of the two components may be adapted to have identical or nearly identical thermal coefficients of expansion. In this manner, the mechanical and geometric changes resulting from temperature changes may be measured as a difference between the components. In some implementations, because each sensor has the same or similar thermal coefficient of expansion, each sensor may expand or contract in a substantially identical manner. Using appropriate sensor circuitry and/or sensor processing, effects on the electrical properties of either sensor as a result of temperature can be substantially compensated, cancelled, reduced or eliminated.

In some embodiments, a substrate may be positioned or disposed below a surface or layer which receives an input force. The substrate may deflect in response to the input force. A first force-sensitive component may be disposed with respect to one side of the substrate and a second force-sensitive component may be disposed with respect to a second, opposite side of the substrate. Due to their placement with respect to a neutral axis of the substrate, the first force-sensitive component may be placed in compression and the second force-sensitive component may be placed in tension in response to the deflection. In some cases, the relative strain response may be compared to estimate or approximate the input force. For example, in some cases, the second force-sensitive component may be placed in greater tensile strain that the first force-sensitive component is placed in compressive strain. The difference or a comparison between the first and second force-sensitive component may be used to estimate the input force. In some embodiments, the first and second force-sensitive films are subjected to similar temperature conditions and may also be subjected to similar mechanical influences due to their proximity to each other. In some cases, the effect of these influences may be reduced or canceled out and by measuring the difference between or comparing the output of the first and second force-sensitive components.

FIG. 1 depicts an example electronic device 100. The electronic device 100 may include a display 104 disposed or positioned within an enclosure 102. The display 104 may include a stack of multiple elements including, for example, a display element, a touch sensor layer, a force sensor layer, and other elements. The display 104 may include a liquid-crystal display (LCD) element, organic light emitting diode (OLED) element, electroluminescent display (ELD), and the like. The display 104 may also include other layers for improving the structural or optical performance of the display, including, for example, glass sheets, polymer sheets, polarizer sheets, color masks, and the like. The display 104 may also be integrated or incorporated with a cover 106, which forms part of the exterior surface of the device 100. Example display stacks depicting some example layer elements are described in more detail below with respect to FIGS. 2-5.

In some embodiments, a touch sensor and or a force sensor are integrated or incorporated with the display 104. In some embodiments, the touch and/or force sensor enable a touch-sensitive surface on the device 100. In the present example, a touch and/or force sensor are used to form a touch-sensitive surface over at least a portion of the exterior surface of the cover 106. The touch sensor may include, for example, a capacitive touch sensor, a resistive touch sensor, or other device that is configured to detect the occurrence and/or location of a touch on the cover 106. The force sensor may include a strain-based force sensor similar to the force sensors described herein.

In some embodiments, each of the layers of the display 104 may be adhered together with an optically transparent adhesive. In other embodiments, each of the layers of the display 104 may be attached or deposited onto separate substrates that may be laminated or bonded to each other. The display 104 may also include other layers for improving the structural or optical performance of the display, including, for example, glass sheets, polarizer sheets, color masks, and the like.

FIG. 2A depicts a top view of an example force-sensitive structure 200 including a grid of optically transparent force-sensitive components. The force-sensitive structure 200 may be integrated or incorporated with a display of an electronic device, such as the example described above with respect to FIG. 1. As shown in FIG. 2A, the force-sensitive structure 200 includes a substrate 210 having disposed upon it a plurality of independent force-sensitive components 212. In this example, the substrate 210 may be an optically transparent material, such as polyethylene terephthalate (PET). The force-sensing components 212 may be made from transparent conductive materials include, for example, polyethyleneioxythiophene (PEDOT), carbon nanotubes, graphene, gallium zinc oxide, indium gallium zinc oxide, other metal oxides, semiconductor material, silver, nickel, or platinum nanowires, other metallic nanowires, and the like. In some cases, the force-sensing components 212 may be formed from a metal oxide, indium oxide, or indium-tin oxide (ITO) material. As previously discussed, the ITO may be formed by doping an indium oxide material with tin (Sn). In some cases, the indium oxide is doped with Sn to a proportion of less than 5%. In certain embodiments, the force-sensing components 212 may be selected at least in part on temperature characteristics. For example, the material selected for the force-sensing components 212 may have a negative temperature coefficient of resistance such that, as temperature increases, the resistance decreases.

As shown in FIG. 2A, the force-sensing components 212 may be formed as an array of rectilinear pixel elements, although other shapes and array patterns could also be used. In many examples, each individual force-sensing component 212 may have a shape and/or pattern that depends on the location of the force-sensing component 212 within the array. For example, in some embodiments, the force-sensing component 212 may be formed as a serpentine pattern of traces, such as shown in FIG. 2B. The force-sensing component 212 may include at least two electrodes 212 a, 212 b for connecting to a sensing circuit. In other cases, the force-sensing component 212 may be electrically connected to sense circuitry without the use of electrodes. For example, the force-sensing component 212 may be connected to the sensing circuitry using conductive traces that are formed as part of the component layer.

FIG. 3 depicts a cross-sectional view of a portion of a device taken along section A-A of FIG. 1. In particular, FIG. 3 depicts a cross-sectional view of an example force-sensitive structure 300 that may be integrated with a display stack and/or a cover of a device. As depicted in the cross-sectional view, a substrate 310 has a first and second force-sensing components 311, 312 disposed on opposite sides of the substrate 310. In this example, the substrate 310 is disposed below a force-receiving layer 320 as part of the display stack. The force-receiving layer 320 may be formed from of a material such as glass. In some cases, the force-receiving layer 320 may be formed from a sapphire sheet, polycarbonate sheet, or other optically-translucent and structurally rigid material. In some embodiments, the force-receiving layer 320 also function as a protective cover element for the device and display stack (e.g., cover 106 of FIG. 1). In some cases, the force-receiving layer 320 may be made from a material having high strain transmission properties. For example, the force-receiving layer 320 may be made from a hard or otherwise rigid material such as glass or metal such that an exerted force may be effectively transmitted through the force-receiving layer 320 to the layers disposed below. The force-receiving layer 320 may also be flexible and able to bend or deflect in response to the force of a touch on the device.

As shown in FIG. 3, the force-sensitive structure 300 includes a substrate 310 having a first force-sensitive component 311 disposed relative to a first, top side of the substrate 310. As shown in FIG. 3, a second force-sensitive component 312 is disposed relative to a second, bottom side of the substrate 310. In this example, the first and second force-sensitive components 311, 312 are formed on or attached to opposite faces of the substrate 310. However, in alternative embodiments, the first and second force-sensitive components may be attached to, or formed on, multiple substrates or layers that are laminated together in the stack.

As shown in FIG. 3, the display stack includes multiple other layers that are disposed between the force-receiving layer 320 and the substrate 310. In particular, the present configuration includes a display element 330 and a polarizer layer 324 that are bonded to a rear face of the force-receiving layer 320 by optically clear adhesive layer 322. As shown in FIG. 3, a rear polarizer 332 is disposed on a side of the display element 330 that is opposite to the force-receiving layer 320. The substrate 310 and first force-sensitive components 311 are attached to the rear polarizer 332 by an optically clear adhesive layer 334. The second force-sensitive components 312 are disposed relative to a side of the substrate 310 that is opposite to the first force-sensitive components 311. Additional structural and/or electrical components 340 may also be disposed below the substrate 310 and the second force-sensitive components 312. The additional structural and/or electrical components 340 may include structural supports and/or shielding to isolate the stack from the other internal electronics of the device. In some cases, the additional structural and/or electrical components 340 may include additional sensor or other electrically active components.

The display stack of FIG. 3 is provided as one specific example. However, the number of layers and the composition of the layers may vary depending on the implementation and the type of display element that is used. For example, in alternative embodiments, the substrate 310, the first force-sensitive components 311 and the second force-sensitive components 312 may be disposed between the display element 330 and the force-receiving layer 320. Independent of the particular composition of the display stack, it may be advantageous that many or all of the layers disposed between the force-receiving layer 320 and the substrate 310 are substantially rigid or be made from materials having high strain transmission properties.

In this example, the force-sensitive components 311, 312 may be formed as an array of rectilinear pixel elements. For example, each force-sensitive component 311, 312 may be aligned vertically (or horizontally) with a respect to a column (or row) of other force-sensitive components 311, 312. In many examples, each individual force-sensing component 311, 312 may be formed in a particular shape or pattern. For example, in certain embodiments, the force sensing component 311, 312 may be deposited in a serpentine pattern, similar to the serpentine pattern shown for force sensing component 212 in FIG. 2B. Other example shapes are described below with respect to FIGS. 5A-D and 7A.

The force-sensitive components 311, 312 are typically connected to sense circuitry 305 that is configured to detect changes in an electrical property of each of the force-sensitive components 311, 312. In this example, the sense circuitry 305 may be configured to detect changes in the resistance of the force-sensitive component 311, 312 which can be used to estimate a force that is applied to the device. In some cases, the sense circuitry 305 may also be configured to provide information about the location of the touch based on the relative difference in the change of resistance of the force-sensitive components 311, 312.

The sensing circuitry 305 may be adapted to determine a difference between a force experienced by the force-sensitive component 311 and the force experienced by the force-sensitive component 312. For example, as described above, a force may be received at the force-receiving layer 320. As a result of the rigidity of the force-receiving layer 320, the force received may be effectively transferred to the substrate 310. Because the force-sensitive components 311, 312 are affixed to the substrate 310, the force-sensitive components 311, 312 experience the force as well. Due to the geometry of the display stack, a deflection in the substrate 310 may result in a lower strain in the first force-sensitive components 311 as compared to the strain in the second force-sensitive components 312. Additionally, the first force-sensitive components 311 may be placed in a compressive strain mode as compared to the second force-sensitive components 312, which may be placed in a tensile strain mode.

In some embodiments, the sensing circuitry 305 may be configured to detect the relative difference in the output of the first and second force-sensitive components 311, 312 to reduce or eliminate the effects of temperature and thermal expansion. In particular, due to their proximity to the same substrate 310, the first and second force-sensitive components 311, 312 may be subjected to similar thermal conditions. In some cases, the first and second force-sensitive components 311, 312 may be at approximately the same temperature and subjected to substantially the same thermal expansion conditions. However, when a force is applied to the force-receiving layer 320, the relative strain between the first and second force-sensitive components 311, 312 may differ. As discussed above, the amount of strain experienced by the second force-sensitive components 312 may be greater than the strain experienced by the first force-sensitive components 311. Additionally the mode of the strain (compressive vs. tensile) will be different between the first and second force-sensitive components 311, 312. In some configurations, the sensing circuitry 305 may be configured to detect the relative difference in the strain in the first and second force-sensitive components 311, 312 and reduce the impact of thermal and other mechanical influences on a force measurement.

In one example embodiment, the sensing circuitry 305 may be configured to measure the relative change in resistance between the first and second force-sensitive components 311, 312. In particular, the first and second force-sensitive components 311, 312 may be resistive elements electrically connected as a voltage divider. In certain examples the first force-sensitive component 311 may be configured as the ground-connected resistor R_(ground) of the voltage divider and the second force-sensitive component 312 may be configured as the supply-connected resistor R_(supply) of the voltage divider. In accordance with some embodiments, the voltage V_(out) at the midpoint of the first force-sensitive component 312 and the second force-sensitive component 312 may be calculated by multiplying the supply voltage V_(supply) by the ratio of the ground-connected resistor to the total resistance (i.e., supply-connected resistor R_(supply) summed with the ground-connected resistor R_(ground)). In other words, the voltage at the midpoint of the voltage divider, V_(out) may be found, in a simplified example, by using the equation:

$\begin{matrix} {V_{out} = {{V_{supply}\left( \frac{R_{ground}}{R_{ground} + R_{supply}} \right)}.}} & {{Equation}\mspace{14mu} 1} \end{matrix}$

Due to fact that the resistance of resistive elements R_(ground) and R_(supply) (or first force-sensitive component 311 and second force-sensitive component 312, respectively) changes in response to force and in response to temperature, the resistance of either element may be calculated as a function of both force (i.e., strain) and as a function of temperature, using as a simplified example, the equation:

R _(measured) ≅R _(baseline)(1+α·(T _(actual) −T _(baseline)))(1+g·ε _(applied)),  Equation 2

where R_(baseline) a baseline reference resistance, α is the temperature coefficient of resistance, g is the strain coefficient of resistance, and ε_(applied) is the strain applied to the structure. The approximation described by Equation 2 illustrates that the base resistance R_(baseline) of either R_(ground) and R_(supply) may be altered by both the temperature and the strain applied to the material. In some cases, the effects of temperature changes may be approximated by the product of the temperature coefficient of resistance a of the material selected for the force-sensitive component, and the difference between the actual temperature T_(actual) of the element and a baseline temperature T baseline. Similarly, the effect of strain may be approximated by the product of the strain coefficient of resistance g and the strain applied ε_(applied) to the element.

By combining Equation 2 and Equation 1 and entering the known quantities V_(supply), R_(baseline), α, and g and measured quantities V_(out), the strain applied to each element ε₂₁₁ and ε₂₁₂ and the actual temperature of each element T₂₁₁ and T₂₁₂ are the only remaining unknown variables, which may be further simplified as a difference in strain Δε between the force-sensitive components 311, 312 and a difference in temperature ΔT between the force-sensitive components 311, 312. As discussed previously, the substrate 310 may be sufficiently thermally conductive to substantially normalize or the temperature between the force-sensitive components 311, 312. Thus, in some cases, the difference in temperature ΔT may be practically approximated as zero. Additionally, the strain relationship between the force-sensitive components 311, 312 may be known due to the physical constraints of the display stack and the substrate 310. For example, knowing the thickness of the substrate 310 and one or more boundary conditions, a strain relationship between the force-sensitive components 311, 312 may be determined. Thus, strain ε₂₁₁ may be expressed in terms of strain ε₂₁₂ or, alternatively, strain ε₂₁₂ may be expressed in terms of strain ε₂₁₁. Accordingly, ε₂₁₁ or (ε₂₁₂) may be solved for and passed to an electronic device or used to calculate an estimate of the applied force. As discussed previously, the force measurement or estimate may be used as a user input for the electronic device.

In practice, a piezoresistive elements of force-sensitive components 311, 312 may be effected as the temperature of the sensor, device, or environment changes. As discussed above, the electrical properties of the force-sensitive components 311, 312 (e.g., resistance) may change substantially with changes in temperature. Additionally, in some examples, the electrical properties of force-sensitive components 311, 312 may also be impacted by the coefficient of thermal expansion (“CTE”) as a result of temperature of the device. Similarly, one may appreciate that the strain-sensitive material of the force-sensitive components 311, 312 may be subject to the changes in resistance due to the changes in temperature. Such changes may be referred to as changes resulting from the thermal coefficient of resistance (“TCR”) of the material selected for force-sensitive components 311, 312. In this manner, the electrical properties of the force-sensitive components 311, 312 may be modeled as the sum of the pyroelectric effect, the CTE effect, the TCR effect, and the effect of any strain as a result of a force applied by a user. Thus, the strain measured directly from the force-sensitive components 311, 312 may be approximated, in one example, as a sum of three components.

ε_(measured)≅ε_(user)+ε_(pyro)+ε_(CTE)+ε_(TCR),  Equation 3

where strain ε_(user) is due to the force of a touch, strain ε_(pyro) is due to pyroelectric effects of the temperature of the sensor, strain ε_(CTE) is strain caused by thermal expansion, and apparent strain ε_(TCR) is strain due to the TCR effect. However, as discussed above, the force-sensitive components 311, 312 may be subjected to similar thermal and physical conditions, and therefore the strain due to effects other than the force of a touch (ε_(user)) may be accounted for by calculating a force measurement based on the relative values of the force-sensitive components 311, 312. For example, the strain ε_(user) may be estimated or approximated using a measurement across the force-sensitive components 311, 312 and a half bridge sensing configuration. Accordingly, in order to facilitate measuring the force applied by the user, the pyroelectric effect and the CTE effect may be reduced, cancelled, eliminated, or otherwise compensated.

In some cases, the sensor circuitry 305 may be configured to reduce or compensate for effects due to other components or electrical influences. For example, in some instances, one or more of the force-sensitive components 311, 312 may electrically couple to one or more other components in the device. With respect to the example depicted in FIG. 3, in some cases, the second force-sensitive component 312 may capacitively couple to the additional structural and/or electrical components 340 disposed below the second force-sensitive components. In one example, the additional structural and/or electrical components 340 include a charged conductive element that may capacitively couple to one or more of the second force-sensitive components 312. As a result, the second force-sensitive components 312 may carry a residual positive or electrostatic charge caused by the capacitive coupling to one or more other components. In some cases, this positive charge may affect the strain-based measurement. Therefore, in some cases, the sensor circuitry 305 may be configured to apply a voltage bias to compensate for the positive charge on one or more of the second force-sensitive components 312. In one example implementation, the second force-sensitive components 312 are virtually grounded or held to a reference voltage to reduce or eliminate the effects of a positive (or negative) charge caused by other electrical components in the device.

In some cases, the sensor circuitry 305 may be configured to selectively sample the electrical response of the force-sensitive components 311, 312. For example, the touch of a user's finger may cause a momentary spike in the electrical output of the force-sensitive components 311, 312. The spike may be due to a sudden induced charge or discharge caused by the touch and/or initial deflection of the substrate 310. Similarly, a spike in the output may be created when the touch is removed and/or the substrate 310 is returned to an un-deflected state. Thus, in some implementations, the sensor circuitry 305 may discard or ignore electrical output of the force-sensitive components 311, 312 at or near a time period associated with an initial touch and/or a time period associated with a removal of a touch. In some cases, the sensor circuitry 305 may be configured to detect a spike by comparing the output to an average or normalized output. The detected spike or spikes may then be discarded or ignored by the sensor circuitry 305 when estimating the applied force.

In some cases, the sensor circuitry 305 may be configured to provide a bias for the electrical response of the force-sensitive components 311, 312. For example, a user's touch may cause a momentary spike in the electrical output of the force-sensitive components 311, 312. The spike may be due to a induced charge or discharge caused by the initial deflection of the substrate 310. Similarly, a spike in the output may be created when the substrate 310 is returned to an un-deflected state. Thus, in some implementations, the sensor circuitry 305 may be biased to a common mode voltage, which may substantially reduce the induced charge and voltage change due to deflection of the substrate up or down. For example, the sensor circuitry 305 may include a voltage divider bridge and voltage on a bridge-connected pair of strain elements may be positive with respect to an element below the sensor. In this case, a positive charge may be induced on the sensor, and if the sensor is deflected downward, the voltage measured by the sensor is reduced for a time constant depending on its resistance and capacitances to other elements. Similarly, if the bridge is biased sufficiently negative with respect to an element below the sensor, an negative charge is induced on the sensor, and the opposite occurs. In some implementations, the sensor may be biased in between a spikes created by both deflection and un-deflection to reduce the effect of the induced charge and voltage transient due to deflections.

In some cases, the sensor circuitry 305 may also be configured to selectively scan the force-sensitive components 311, 312 to improve the sensitivity or responsiveness of the sensor. For example, the sensor circuitry 305 may be configured to receive location information associated with the touch location on the touch on the force-receiving layer 320. In some cases, the location information may be provided by a separate touch sensor. In some cases, the location information may be provided by the force sensor itself. The location information may be used to increase the number of measurements or sensor reading events for a selected subset of force-sensitive components 311, 312 that correspond to the touch location. For example, if the sensor circuitry 305 is configured to measure the force-sensitive components 311, 312 using a repeating scan sequence, the number of force-sensitive components 311, 312 may be reduced to a selected subset that may correspond to the location of a touch. By reducing the number of force-sensitive components that are scanned, scan rate may be increased thereby increasing the number of measurements that may be read for the selected subset. The remaining force-sensitive components 311, 312 may also be periodically scanned, but at a rate that is lower or reduced with respect to the selected subset. By increasing the number of measurements for force-sensitive components that correspond to a touch location, the overall responsiveness and accuracy of the sensor may be improved.

In some cases, the force-sensitive structure 300 may be configured to reduce or eliminate the effects of temperature on an interconnect formed from dissimilar materials. For example, in some cases the force-sensitive components 311, 312 may be electrically connected to the sensor circuitry 305 via one or more electrical interconnects. In one particular configuration, the first force-sensitive components 311 are connected to the sensor circuitry via a first interconnect and the second force-sensitive components 312 are connected to the sensor circuitry via a second interconnect. In some cases, the first and second interconnects include the electrical connection of two or more dissimilar materials. For example, the interconnects may be formed from an ITO to aluminum electrical connection. In some cases, the interconnects may be formed from an ITO to silver paste to copper electrical connection. Generally, connecting two dissimilar material may generate a temperature-dependent voltage, which may affect the force measurement. Therefore, in some instances, it may be advantageous that the first interconnect and the second interconnect be formed from the same set of dissimilar materials, and the junctions placed in proximity, to cancel or reduce the effects of temperature. In some instances, the sensor circuitry 305 may also be configured to compensate for the effects of a temperature-dependent voltage on the force measurement. For example, the sensor circuitry may be configured to measure or estimate the junction temperatures and perform a compensation calculation. The compensation calculation may account for dissimilar materials used in the interconnect and associated junctions.

FIG. 4 depicts a top view of an alternate example of a force-sensitive structure 400 including two angularly-offset layers, each including multiple optically transparent force-sensitive components 412, 422. One of the layers may be arranged as a number of rows while the other is arranged as a number of columns. Further, one of the sets of rows and/or columns may be electrically driven while the other set is a sense layer. As noted with respect to FIG. 2A, other suitable configurations of transparent force-sensitive components are contemplated. For example, the angular offset between the layer 412, 422 may be perpendicular in certain embodiments and a different angle in other embodiments.

FIGS. 5A-5C depict a top detailed view of various optically transparent serpentine geometries for a force-sensitive component which may be used for either or both of the example force-sensitive structures depicted in FIG. 3 (311, 312). In the present example, the force sensing component 512 may include at least two electrodes 512 a, 512 b for connecting to a sensing circuit. However, in some cases, the force sensing component 512 may be electrically connected to sense circuitry without the use of electrodes. For example, the force sensing component 512 may be connected to the sense circuitry using conductive traces that are formed as part of the component layer.

FIG. 5A depicts a top view of a serpentine geometry which is sensitive to strain along the Y-axis. In this manner, when the force-sensing component 512 is strained along the X-axis direction, the force-sensing component 512 may not experience substantial tension or strain response. Conversely, when the force-sensing component 512 is strained along the Y-axis direction, a strain may be detected and measured. One may appreciate that angular strain (e.g., strain along a 45 degree path) may strain the force-sensing component 512 in some amount that may be detected and measured. Similarly, FIG. 5B depicts a top view of a serpentine geometry which is sensitive to strain along the X-axis, and may not be particularly sensitive to strain along the Y-axis. FIG. 5C depicts a top view of a serpentine geometry which may be sensitive to strain along both the X and Y axis.

FIG. 5D depicts a top view of a serpentine geometry which may be sensitive to strain along a 45 degree angle. One may appreciate that although shown at 45 degrees, that any angle or combination of angles may be employed. For example, one embodiment may include angling a strain sensor 512 along an 80 degree angle. Another embodiment may include a strain sensor having multiple distinct portions similar to FIG. 5C, in which one portion is angled at 25 degrees and another portion is angled at 75 degrees. In many embodiments, the angle or combination of angles of orientation for different force-sensitive components may be selected, at least in part based on the location of the particular force-sensitive component along the surface of an electronic device.

For example, FIG. 6 depicts a top view of an example force-sensitive structure including an array or grid of optically transparent force-sensitive components 612 a-c oriented to detect force in different directions. For example, force-sensitive component 612 a may be oriented to detect strain along a 45 degree angle, whereas force-sensitive component 612 b may be oriented to detect strain along a −45 degree angle. In another examples, force-sensitive component 612 c may be adapted to detect along an arbitrary angle between 0 and 45 degrees. The force-sensitive components 612 a-c may be used to form a force sensor in accordance with one or more of the embodiments described herein. In particular, an array or grid of force-sensitive components similar to those depicted in FIG. 6 may be used for either or both of the force-sensitive components 311, 312 discussed above with respect to FIG. 3.

In certain embodiments, the material of the force-sensitive components may be sensitive to strain in the plane of the sensor along two or more primary axes by producing an output having the same polarity. For example, a force-sensitive component made of ITO or other metal oxides, or polysilicon, may respond to strain in both the primary axes with a charge having the same sign, and thus may be sensitive to the sum of the net strains. In some cases, the force-sensitive structures may include traces arranged in serpentine pattern that be configured to spatially sample and average strain produced along a particular direction, and/or be configured to have a resistance within a desired range.

In certain embodiments, the force-sensitive components may have a resistance that is configured to maximize the signal to noise ratio with respect to the power and voltage that is available on the device. In some cases, it may also be desirable to use high-resistivity material in order to improve the optical properties of transparent conductors. The force-sensitive components may be configured to balance or satisfy these constraints. For example, a force-sensitive component may be formed from an ITO or other metal oxides, or polysilicon material having in a geometry to provide a specific resistance. In some cases, the serpentine may be configured to spatially sample and average the desired strains. In some cases, the serpentine and conductive material may be configured to provide a resistance or electrical response that falls within a range that satisfies constraints on the response time of the sensor, the optical properties of the sensor, and the noise contributed by the sensor and interfaces to the material of the sensor. In some cases, the desired resistance of the force-sensitive component may be in the range of 20,000 ohms to 200,000 ohms, which may be higher than the typical strain gauge sensor for other applications.

In certain embodiments, the orientation of force-sensitive components may be selected based on the position of the force-sensitive component relative to the housing of an electronic device. For example, a force-sensitive component positioned proximate to the edge of a screen within a display stack may be oriented differently from a force-sensitive component positioned in the center of the display.

In some embodiments, as shown in FIG. 6, the grid may be formed from an array of components that includes a subset of edge force-sensitive components 612 c positioned along an edge of the first array. In some cases, the edge force-sensitive components 612 c are formed from traces that are oriented along a direction that is substantially perpendicular to the edge. As shown in FIG. 6, the array of force-sensitive components may include a subset of corner force-sensitive components 612 a, 612 b positioned at corners of the array or grid. In some cases, the corner force-sensitive components 612 a, 612 b are formed from traces that are oriented along a diagonal direction.

FIG. 7A depicts another example force-sensitive component 712 that can be used in a force sensor. As shown in FIG. 7A, the force-sensitive component 712 includes traces that are substantially oriented in two primary directions. In particular, a first portion 712 a of the force-sensitive component includes traces that are substantially oriented along a y-direction and a second portion 712 b includes traces that are substantially oriented along an x-direction. In this particular example, two first portions 712 a are arranged adjacent to two second portions 712 b to form an alternating pattern. This configuration may be advantageous in sensing strain in both the x- and y-directions. This configuration may be further advantageous by increasing the resolution or sensitivity of each force-sensitive component, as compared, for example to an arrangement having traces aligned along a single direction.

FIG. 7B depicts a top view of an example force-sensitive structure 700 formed from an array or grid of force-sensitive structures 712. The force-sensitive structure 700 having an arrangement of force-sensitive components 712 may be used to form a force sensor in accordance with one or more of the embodiments described herein. In particular, an array or grid of force-sensitive components 712 as depicted in FIG. 7B may be used for either or both of the force-sensitive components 311, 312 discussed above with respect to FIG. 3.

FIG. 8 depicts a simplified signal flow diagram of a temperature-compensating and optically transparent force sensor in the form of a Wheatstone bridge. In such an embodiment, a voltage Vg may be measured across the output of two parallel voltage dividers connected to a voltage supply Vs. One of the voltage dividers may include two resistors of known resistance R₃, R₄ and the other voltage divider may include two variable resistors R₃₁₂, R₃₂₂ that model the force and temperature variable resistance of the force-sensitive components 311, 312 as shown, for example in FIG. 3. By combining, for example, Equation 2 and Equation 1 after entering the known quantities V_(supply), R_(baseline), α, g, R₃, and R₄ and measured quantities V_(out), the strain ε₂₁₂ applied to the force-sensitive component 312 may be the only remaining unknown and accordingly may be determined using, for example, the circuit depicted in FIG. 8 and potentially other electronics or processors. The output of the circuit depicted in FIG. 8 may be passed to another component of an electronic device and/or used to compute a force measurement.

FIG. 9 is a process flow diagram illustrating example operations of a sample process 900 of manufacturing a temperature-compensating and optically transparent force sensor. The process 900 may begin at operation 902 in which a substrate may be selected or obtained. The substrate may include, for example, glass or transparent polymers like polyethylene terephthalate (PET) or cyclo-olefin polymer (COP). After the substrate is obtained, a force-sensitive component may be applied thereto at operation 904. In one example, an array of force-sensitive components or pixels are formed on a first surface of the substrate. The force-sensitive components may be formed from a transparent conductive material, including, for example, polyethyleneioxythiophene (PEDOT), metal oxide, indium oxide, indium-tin oxide (ITO), carbon nanotubes, graphene, semiconductor material, silver nanowire, other metallic nanowires, and the like. In some cases, the thickness of the transparent conductive material may be substantially increased if the substrate is formed from a glass material, which may tolerate higher temperature manufacturing processes. For example, a transparent conductive material that is grown at higher temperatures may exhibit lower sheet resistance and may be used to form a more sensitive or robust force sensor. During manufacturing, higher thermal conductivity substrates, such as glass, may allow for use of thicker substrates while maintaining a substantially equal temperatures on both sensor planes. For example, in some embodiments, the substrate may have a thermal conductivity of greater than 0.5 Watts per square meter per degree Kelvin. In some cases, the force-sensitive component is formed directly onto the substrate. In other cases, the force-sensitive component is formed on another material or substrate, which is bonded or otherwise attached to the substrate.

In operation 906 another (second) force-sensitive component may be applied to a second surface of the substrate. In some cases, the second surface is opposite to the first surface. The second force-sensitive component may be formed from the same or a similar transparent conductive material as used in operation 904. One advantage to using the same transparent conductive material is that effects due to temperature and thermal conditions may be compensated, reduced, or eliminated using, for example, some of the techniques described herein. Similar to operation 904, the second force-sensitive component may be formed directly onto the substrate. In other cases, the second force-sensitive component may be formed on another material or substrate, which is bonded or otherwise attached to the substrate. In general, it should be appreciated that the order of operations may vary between embodiments.

FIG. 10 is a process flow diagram depicting example operations for a process 1000 of operating a temperature-compensating force sensor. Process 1000 may be used, for example, to operate one or more of the force sensors described with respect to FIG. 3, above. In particular, process 1000 may be used to compute or estimate the force of a touch on a device and compensate for variations or effects of temperature.

In operation 1002, an occurrence of a user touch may be detected. The touch may be detected, for example using a touch sensor. The touch sensor may include, for example, a self-capacitive, mutually capacitive, resistive, or other type of touch sensor. In some embodiments, the occurrence of a touch may be detected by the force sensor. For example, a change in strain or resistance or strain of one or more force-sensitive structures of the sensor may be used to detect the occurrence of a touch. In some embodiments, operation 1002 is not necessary. For example, the other operations of process 1000 may be performed on a regularly repeating or irregular interval without first determining if a touch is present. For example, process 1000 may be performed and calculate or estimate a zero applied force, which may be due to the absence or lack of a touch on the device.

In operation 1004, a relative measurement between two or more force-sensitive structure may be obtained. As described previously with respect to, for example, FIGS. 3 and 8 a relative measurement may be obtained using a voltage divider, half bridge, full bridge, or other similar circuit configuration. In some embodiments, an electrical measurement of each individual force-sensitive structure is obtained and the measurements are compared using software, firmware, or combination of software/firmware and circuit hardware.

In operation 1006, a force estimate may be computed. In some embodiments, the force estimate compensates for variations in thermal effects, including, for example a pyroelectric effect, TCR effect, and/or CTE effect, as described above with respect to Equation 3. In particular, the relative measurement obtained in operation 1004 may be used in combination with Equations 1 and 2 to compute an estimated strain. The estimated strain may then be used to estimate an applied force using, for example, a known correlation between the strain of the corresponding force-sensitive structure and an applied force. For example, the strain may correspond to an estimated deflection of the substrate (and other relevant layers of the display/sensor stack), which may correspond to a respective force on a surface of the device.

FIG. 11 is an additional process flow diagram illustrating example steps of a process 1100 for operating a temperature-compensating force sensor. In operation 1102, a location of a user touch may be identified. The location of a user touch may be determined, using for example a self-capacitive touch sensor, a mutually capacitive touch sensor, a resistive touch sensor, and the like.

In operation 1104 a relative measurement between two or more force-sensitive structure may be obtained. As described previously with respect to, for example, FIGS. 3 and 8 a relative measurement may be obtained using a voltage divider, half bridge, full bridge, or other similar circuit configuration. In some embodiments, an electrical measurement of each individual force-sensitive structure is obtained and the measurements are compared using software, firmware, or combination of software/firmware and circuit hardware.

In operation 1106, a force centroid is calculated. For example, the relative measurement obtained in operation 1104 may be used to approximate the centroid of the applied force at 1106. In some embodiments, the location of the user touch obtained in operation 1102 may be used to approximate the centroid of the applied force. In some embodiments, the geometric centroid of all touches of a multi-touch event may be used to approximate the centroid of the applied force. Thereafter, the measured force and the force centroid may be forwarded or otherwise relayed to the electronic device in operation 1108.

One may appreciate that although many embodiments are disclosed above with respect to optically transparent force sensors, that the systems and methods described herein may apply equally well to opaque force sensors or force sensors that are not required to be transparent. For example, the force sensors described herein may be included below a display stack, or within the housing of a device. For example, an electronic device may be adapted to react to a user squeezing or applying pressure to a housing of an electronic device. Such a force sensor need not, in all embodiments, be transparent. Still further embodiments may include a force sensor that is translucent. For example, a force sensor component may be doped with an ink such that the force sensor appears as a particular color or set of colors. In still further embodiments, the force sensor may be optionally transparent, translucent or opaque.

Embodiments described herein may be formed in any number of suitable manufacturing processes. For example, in one embodiment, a force-sensitive structure may be formed in a roll-to-roll process which may include depositing a force-sensitive material in a selected pattern on a substrate, bonding said substrate to one or more additional layers or components of an electronic device, and singulating the output of the roll-to-roll process into a plurality of independent force-sensitive structures.

One may appreciate that although many embodiments are disclosed above, that the operations and steps presented with respect to methods and techniques described herein are meant as exemplary and accordingly are not exhaustive. One may further appreciate that alternate step order or, fewer or additional steps may be required or desired for particular embodiments.

Although the disclosure above is described in terms of various exemplary embodiments and implementations, it should be understood that the various features, aspects and functionality described in one or more of the individual embodiments are not limited in their applicability to the particular embodiment with which they are described, but instead can be applied, alone or in various combinations, to one or more of the other embodiments of the invention, whether or not such embodiments are described and whether or not such features are presented as being a part of a described embodiment. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments but is instead defined by the claims herein presented. 

We claim:
 1. An optically transparent force sensor comprising: a force-receiving layer; a substrate disposed below the force-receiving layer and formed from an optically transparent material a first force-sensitive component disposed on a first side of the substrate and formed from an optically transparent, strain-sensitive material; and a second force-sensitive component disposed on a second side of the substrate that is opposite to the first side, wherein the second force-sensitive component is formed from the optically transparent, strain-sensitive material.
 2. The force sensor of claim 1, further comprising: sensor circuitry that is operatively coupled to the first force-sensitive component and the second force-sensitive component, wherein the sensor circuitry is configured to measure a relative difference between an electrical response of the first and second force-sensitive components in response to a force of a touch on the force-receiving layer, and compute a temperature-compensated force estimate using the relative difference.
 3. The force sensor of claim 1, wherein the substrate is configured to conduct heat between the first force-sensitive component and the second force-sensitive component to achieve a substantially uniform temperature distribution.
 4. The force sensor of claim 1, wherein the substrate is disposed below a display element of an electronic device.
 5. The force sensor of claim 1, wherein the substrate is disposed between a cover and a display element of an electronic device.
 6. The force sensor of claim 1, wherein the first force-sensitive component is placed in compression in response to a force of a touch on the force-receiving layer, and the second force-sensitive component is placed in tension in response to the force of the touch.
 7. The force sensor of claim 1, further comprising: a first array of rectilinear force-sensitive components including the first force-sensitive component; and a second array of rectilinear force-sensitive components including the second force-sensitive component.
 8. The force sensor of claim 1, wherein the force-receiving layer is a cover of a display of a device and is formed from a glass material.
 9. The force sensor of claim 1, wherein the first and second force-sensitive components have substantially identical temperature coefficients of resistance.
 10. The force sensor of claim 1, wherein the substrate has a thermal conductivity of greater than 0.5 Watts per square meter per degree Kelvin.
 11. The force sensor of claim 1, wherein the first and second force-sensitive components are formed from a piezoresistive material.
 12. The force sensor of claim 1, wherein the first and second force-sensitive components are formed from one or more of: a carbon nanotube material, graphene, a semiconductor material, a metal oxide material.
 13. The force sensor of claim 1, wherein the first and second force-sensitive components are formed from a indium oxide material that is doped with Sn.
 14. The force sensor of claim 13, wherein the indium oxide material doped with Sn to a proportion of less than 5%.
 15. An electronic device having an optically transparent force sensor comprising: a cover; a substrate disposed below the cover and formed from an optically transparent material; a first array of force-sensitive components disposed on a first side of the substrate and formed from an optically transparent, strain-sensitive material; and a second array of force-sensitive components disposed on a second side of the substrate that is opposite to the first side, wherein the second array of force-sensitive components is formed from the optically transparent, strain-sensitive material sensor circuitry that is configured to compare a relative electrical response between respective components of the first array of force-sensitive components and the second array of force-sensitive components, and configured to compute a temperature-compensated force estimate.
 16. The electronic device of claim 15, further comprising: a display element disposed above the first array of force-sensitive components.
 17. The electronic device of claim 15, further comprising: a display element disposed below the second array of force-sensitive components.
 18. The electronic device of claim 15, wherein the first array of force-sensitive components includes a subset of edge force-sensitive components positioned along an edge of the first array, wherein the edge force-sensitive components are formed from traces that are oriented along a direction that is substantially perpendicular to the edge.
 19. The electronic device of claim 15, wherein the first array of force-sensitive components includes a subset of corner force-sensitive components positioned at corners of the first array, wherein the corner force-sensitive components are formed from traces that are oriented along a diagonal direction.
 20. The electronic device of claim 15, wherein the first array of force-sensitive components includes component having a first portion that includes traces that are substantially oriented along a first direction and a second portion that includes traces that are substantially oriented along an second direction, wherein the first direction is substantially perpendicular to the second direction. 